The present invention relates to a method for improving the radionuclide retention properties of solidified radioactive wastes wherein the wastes are present in solid or in aqueous, flowable or liquid form, are encased in or mixed with an inorganic and/or an organic binder or binder mixtures containing at least one additive and are then left to harden.
Radioactive wastes, regardless of their origin and type, must be solidified before their final storage, often even before their intermediate storage, to eliminate the risk of uncontrolled entry of radionuclides into the biocycle or to at least substantially reduce such risk. For 25 years or more, a large number of proposals have been made for solidification processes and for solidification matrices. Low to medium radioactive aqueous wastes, such as, for example, aqueous solutions or aqueous concentrates, precipitation sludges, ashes from the combustion of combustible radioactive wastes and solid, radioactively contaminated parts were mixed with or encased in, inter alia, a cement slurry, with or without additives, and left to harden into solid stone-like blocks or shaped bodies.
Spent inorganic or organic ion exchangers containing radioactive substances have also been treated in this manner, and hardening organic resins or polymerizable organic masses have been used as matrices for radioactive waste materials. For example, R. Alberti discloses in German Pat. No. 1,082,993 that a radioactive aqueous liquid can be solidified in an isodispersive grain mixture of, for example, heavy spar, together with a hardening binder, such as, for example Portland cement, alumina cement or oxychloride cement, or by the addition of a casting resin. This patent already proposes measures to further prevent leaching of the incorporated radionuclides, such as, for example, by providing the solidified shaped body with a corrosion-proof coating of a casting resin or Teflon.
For more than 20 years, the experts in the art have attempted to improve the radionuclide retention properties of solidified radioactive wastes. Particularly in solidifications with cement, with or without aggregates, resulting in cement rock or concrete-like products, only partial successes have been realized without complicated and/or expensive measures. Particularly for the radionuclides, which are the most disturbing to the biocycle, the leaching rates in water or aqueous solutions, from radioactive waste products solidified with cement or the like, are still undesirably high. So far, series of experiments have been described in a large number of publications, in which very many types of cement, sometimes mixed with liquid absorbing materials, such as, for example, clays amd/or rock-like substances, have been examined for their usefulness as solidification matrices for radioactive wastes.
Such solidification processes and the resulting solid shaped waste bodies themselves must meet specific requirements which vary according to state law but are often not yet strictly defined. Examples of such requirements are the international transporting regulations for radioactive materials. In this respect, the requirements to be met by a solidified binder mixture with wastes distributed therein as homogeneously as possible are: (1) water resistance (insolubility and shape retention in water or aqueous solutions); (2) sulfate resistance; (3) radiolysis resistance; (4) the most extensive leaching resistance for the radionuclides bound in the solidification matrix, i.e., very low leaching rates or diffusion constants; and (5) chemical and physical stability over long periods of time.
Boric acid containing or borate containing ion exchanger resins containing spent radioactive substances and/or aqueous solutions or concentrates containing boric acid or borate, respectively, often prove difficult to solidify with cement. In past attempts to solidify such wastes, either no compact, solid blocks resulted which had sufficient pressure resistance, instead only cracked and crumbling masses were produced, or the radioactive liquid was not completely incorporated. Although the addition of waterglass (sodium silicate) has been recommended for the solidification of borate containing solutions, this did not bring the desired success with solutions containing more than 5% borate or suspensions containing moe than 5% solid matter. In no case, however, did such products attain sufficient leaching resistance.
German Offenlegungsschrift No. 2,603,116 discloses a method for solidifying radioactive, boron containing aqueous solutions and suspensions containing more than 5% boric acid or borate and more than 5% solids by adding suitable aggregates in the correct sequence, resulting in solid, transportable and storable blocks. In this proposal there is initially added 5 to 30 parts by weight of slaked (hydrated) lime to 100 parts by weight of radioactive solution and then 30 to 80 parts by weight cement. Leaching tests of solidified blocks obtained in this manner have shown, however, that leaching rates for some radionuclides, particularly for cesium-137, are still undesirably high.